Mostly Sci-Fi and/or fantasy stuff for me. I'll watch re-runs of all Trek series, Stargate... all of those I watched religiously on original air. But just that. Series that I do watch now that are not Sci-Fi like House, for example, I don't re-watch.
The same goes for movies. The Trek movies, the Star Wars movies, the LOTRs... Maybe a few rare comedies as well but that's it.